The Pennsylvania Supreme Court on Monday threw out the state's congressional map as unconstitutionally gerrymandered and gave lawmakers until Feb. 9 to redraw the boundaries, a decision that will have major ramifications for the 2018 midterm elections.

Under a new map, Democrats, who hold only five of the state's 18 congressional districts despite its status as a closely divided swing state, would likely have a much better opportunity to pick up several seats in their quest to retake...